We studied the effects of hypoxia on contractile and endurance properties of respiratory muscles in the developing rat. Wistar rats aged postnatal day (P)19 and P29 were killed humanely and the diaphragm and sternohyoid (a representative pharyngeal dilator) muscles were surgically removed. Isometric contractile properties of isolated muscle strips were measured in tissue baths containing physiological salt solution at 30°C under hyperoxic (95%O2/5%CO2) or hypoxic (95%N2/5%CO2) conditions. Force-frequency relationship and fatigue index (ie ratio of force at 5min of fatigue to initial force) were examined. Fatigue was assessed in response to repeated tetanic contractions (40Hz, 300msec train duration) every 2 sec for 5 minutes. Hypoxia decreased specific force in the sternohyoid muscle but had no effect on diaphragm muscle force (peak force in sternohyoid muscle at P19 was 5.5±0.9 vs. 2.8±0.5*, P29 was 9.2±1.2 vs. 3.6±0.8*, mean±SEM N/cm2 , hyperoxia vs. hypoxia, P<0.05 ANOVA).We found that in vitro hypoxia significantly reduced muscle endurance in both the sternohyoid and diaphragm muscle. We also observed an age-dependent decrease in endurance for both muscles in hyperoxic and hypoxic groups. Thus, sternohyoid fatigue index for P19 was (65.7±4.5% vs. 21.7±2.7%*), P29 was (37.4±2.7% vs. 2.7±1.7%*; mean±SEM hyperoxia vs. hypoxia, P<0.05 ANOVA). Diaphragm fatigue index for P19 was (80.7±2.7% vs. 27.9±2.7%*), P29 was (62±4.5% vs. 19.7±1.8%*, mean±SEM, hyperoxia vs. hypoxia, P<0.05 ANOVA). We conclude that hypoxia impairs respiratory muscle function. Our results suggest that the sternohyoid muscle is more vulnerable to hypoxic insult than the diaphragm muscle. We speculate that this is due to their different fibre type characteristics. The mechanism for hypoxia-induced muscle impairment remains unknown. However, the effects of hypoxia may have implications for the control of airway patency in vivo.